Passenger transporting institutions, such as airlines, railroads, and bus companies, have the duty to transport passengers safely. In particular, accidents involving aircraft can lead to disasters and a very high level of safety is required. Thus, airplane passengers are subjected to various tests, such as baggage inspection using X-ray imaging devices, body check through frisking or using metal detectors, and, if necessary, interrogation, so as to pick out passengers with malicious intent and prevent them from boarding the airplane. However, in view of the large number of passengers and the convenience for them, it is difficult to subject all the passengers to strict inspections over a long time or to interrogations. Meanwhile, passengers with malicious intent (such as terrorists) try to slip through these inspections and bring dangerous objects on board. While there would be no problem as long as such dangerous objects can be detected by the current baggage inspection and the like, there are some objects that are difficult to detect using metal detectors or X-ray imaging devices, such as gasoline and other combustible liquids. Gasoline and other dangerous liquids are easy to obtain on the market. If such a dangerous liquid is contained in a commercially available beverage container (such as a PET bottle), for example, it becomes more difficult to distinguish it from authentic beverages, and someone with sinister intent could readily adopt such technique. Thus, it is necessary to devise and prepare countermeasures against such dangerous acts.
In order to distinguish a dangerous liquid such as gasoline from a beverage that typically consists primarily of water, the liquid could be subjected to a sensory test, such as smelling, or other various methods. However, in the baggage inspection before boarding an airplane, time is of utmost concern and the inspection should be completed as quickly as possible. In response to such needs, the inventors had developed a method for determining the type of liquid in containers made of insulating (dielectric) material, such as PET bottles, based on the difference in dielectric constant that depends on the type of the liquid. The inventions associated with such technique are described in the specification attached to JP Patent Application No. 2003-198046 or 2003-385627 filed by the same applicants as the present application.
Besides the aforementioned method for determining the type of a liquid based on the difference in dielectric constant that depends on the type of liquid, a method is conceivable that takes advantage of the difference in thermal characteristics that depend on the type of liquid. For example, Patent Document 1 discloses a technique involving a heat supply means and a temperature-change measuring means that are disposed inside the fuel tank such as the gas tank of an automobile. In this technique, the nature of the fuel (such as its boiling point and T50 value) inside the tank is detected based on the behavior of heat transmitted to heat conducting members on the side of the wall surface of the tank and on the side of the fuel. Patent Document 2 discloses a technique whereby, in order to detect the introduction of water and the like into a petroleum tank or oil delivery channels, an indirectly heated flow detector is used as a fluid distinguishing device. It is well-known that an indirectly heated flowmeter is a current meter comprised of a heating element and a flow rate detecting element (temperature sensor) that are disposed within the fluid, and that it utilizes the property that the temperature of the flow rate detecting element varies depending on the rate of the fluid. In the technique disclosed in Patent Document 2, the fact that the initial output at rate zero of the indirectly heated flowmeter varies depending on the thermal characteristics of the fluid that is in contact therewith is used for the identification of the fluid. Furthermore, Patent Document 3 discloses a technique involving a level measuring device that utilizes a measurement module equipped with a heating means for heating the outer surface of a container and a temperature sensor disposed in the vicinity of the heating means. In this level measuring device, a plurality of measurement modules are arranged outside the container in a row in a biased manner, and the device aims to detect between which measurement modules the fluid level is at based on the difference in behavior of the heat in the container outer wall when there is liquid in the container and when there is not. These techniques disclosed by Patent Documents 1 to 3 all attempt to distinguish the type of liquid (or the presence or absence thereof) based on the thermal characteristics of the liquid (including when there is no liquid).
Patent Document 1: JP Patent Publication (Kokai) No. 10-325815 A (1998)
Patent Document 2: JP Patent Publication (Kokai) No. 2000-186815 A
Patent Document 3: JP Patent Publication (Kokai) No. 2002-214020 A